1. Field of the Invention
The present invention relates to RF tuners and more particularly relates to programmable tuner circuits that are integrated on a single chip.
2. Background
Radio frequency (RF) tuners for receiving and interpreting broadband signals over coaxial cable have many applications. They are found in cable modems to allow high speed access to Internet data; in cable set top boxes and television receivers to allow a television to receive and decode digital television broadcasts as well as Internet data; in personal computers to provide multi-media capability; and in many other broadband cable access devices. In many of these and other applications, tuners are being called upon to receive and tune both analog and digital signals, and also to receive and tune multiple signals for concurrent display. A cable set top box, for example, may receive analog television signals and digital Internet data. Moreover, the set top box may be required to receive and tune these signals simultaneously.
Although a single tuner is often capable of receiving and tuning both analog and digital signals, it is usually not optimized for both types of signals. In a tuner employing a known superheterodyne “dual” or “double” conversion architecture, for example, more power is consumed for analog reception than for digital reception. Analog reception requires more power than digital reception because the tuner needs better linearity to maintain lower intermodulation distortion products. Digital reception has less stringent linearity requirements and hence consumes less power than does analog reception. Hence, when digital signals are received and tuned on a tuner optimized for analog operation, power is wasted.
Discrete “canned tuner” solutions have been proposed using a front-end tracking filter and a single conversion architecture to optimize the tuner for lower power consumption. The front-end tracking filter enhances the linearity of the tuner without significant power consumption. The linearity enhancement provided by the tracking filter is sufficient to support both analog and digital reception without a difference in power consumption. The disadvantage to this solution, of course, is that all tuner components are not integrated on a single IC. Alternative solutions provide separate, dedicated chips in the tuner for analog and digital reception.
A dual conversion tuner can be integrated on a single IC, which provides several advantages relative to a discrete single conversion tuner. These include better gain flatness over input frequency, a significantly lower external component count, no manual or automated tuning, and a cheaper tuner solution. A suitable solution has not been found, however, to optimize power consumption in a dual conversion tuner.
In addition to the challenge of mixed analog/digital operation, advanced receivers such as set top boxes and cable modems often require multiple tuners in order to simultaneously receive and support multiple types of broadband signals, such as a main video signal, cable modem Internet access, picture-in-picture, video-on-demand and so on. In a multi-tuner receiver, a single RF input (which may include multiple broadband signals) is split and provided to separate tuners. In this manner, each tuner can extract its desired signal from the RF input (i.e., main video, picture-in-picture, etc.) and the multiple signals may be displayed simultaneously if desired.
A typical multi-tuner receiver receives multiple signals at different frequencies on a single RF input. The input RF signal is amplified by a low noise amplifier (LNA) optimized for multi-tuner operation, and split among multiple paths to separate tuners that each tune and amplify a different band of the RF signal, permitting concurrent receipt and display of multiple signals. Since the front-end performance requirements for the individual tuners differ depending on application, a fully integrated tuner with fixed performance specifications is not practical.
Typically, the LNA in an integrated tuner design is either not integrated or not selectable. Where the LNA is not integrated, the LNA is chosen in order to optimize the particular receiver application. For instance, in a single signal reception environment an LNA with a certain specification is used, and in a multiple signal reception environment an LNA with a different specification is used. Where the LNA is integrated, conversely, it is not selectable. Hence, an LNA optimized for a single reception environment could not be bypassed in a multiple reception environment. Instead, a completely different tuner IC with a different LNA would be necessary. A tuner IC having an integrated and selectable LNA, capable of operating in both a single-tuner and multi-tuner environment, is highly desirable.